Optical collimating apparatus have been known for some time. For example, U.S. Pat. No. 4,704,010 discloses a device capable of generating an optical collimating beam using a single, plano-convex lens. A collimating mark is applied on the convex surface and a reflective coating is applied to the central portion of the plano surface. Light emanating from the collimating mark makes a double pass through the lens, exiting the plano surface as a collimated beam. The collimating mark is imaged at infinity. In a specific embodiment, the collimating mark is illuminated using a prism.
U.S. Pat. No. 4,859,031 discloses an optical collimating device using a semi-reflective concave mirror and a cholesteric liquid crystal element. In at least one of the disclosed embodiments, the collimator is used in conjunction with a combiner, thus allowing the system to be used in a heads-up display device. Images in the line of sight of the viewer substantially pass through the combiner, semi-reflective mirror, and cholesteric liquid crystal to the viewer. Images generated by a source are reflected by the combiner into the line of sight of the viewer. The generated images pass through the semi-reflective mirror to the cholesteric liquid crystal element. The cholesteric element reflects the images back to the concave side of the semi-reflective mirror. The concave mirror creates an image of the source at the same time it reverses the polarization of the image, thus allowing it to pass through the cholesteric element to the viewer.
U.S. Pat. No. 5,408,346 discloses an optical collimating device using a cholesteric liquid crystal element, the device exhibiting improved image transmissivity. Both reflective and transmissive systems are disclosed. The patent also discloses the use of the collimator in conjunction with a combiner, thus allowing multiple images to be superimposed for viewing by the user.
U.S. Pat. No. 5,715,023 discloses a compact display system that can be used as a collimating eyepiece. The system uses a cholesteric liquid crystal element in combination with an optical doublet. A partially reflective coating is at the interface between the two singlets which comprise the doublet. The design of this system reduces the number of element to air and/or element to element interfaces, thus reducing losses and ghosting while making a sturdy, vibration tolerant display system. In one embodiment of the disclosed system multiple cholesteric liquid crystal elements are used, thus achieving a multicolor display system.
From the foregoing, it is apparent that a compact, collimating eyepiece is desired that is capable of operating over a wide range of wavelengths. This device is of particular interest in the area of head-mounted displays where size and weight are critical factors.